1. Field of the Invention
The present invention relates to operational amplifiers and, in particular, to a precision trimming circuit that provides trim current compensation with the necessary temperature coefficient to track changes in input offset voltage with variations in temperature.
2. Discussion of the Prior Art
An operational amplifier (op amp), a basic building block in analog integrated circuits, amplifies the difference between two different potentials. A basic op amp consists of a dc amplifier with a differential input and a single-ended output.
An ideal op amp has zero output voltage for zero input. In reality, however, because of the inherent lack of precision in matching the op amp's two differential input transistors, an op amp will always have some output voltage for no input. This voltage is called the offset voltage. Since the output offset voltage is independent of op amp gain, it can easily be related to the differential input signal levels by dividing the output offset voltage at zero input by the closed loop gain. The result is typically referred to as the "input offset voltage."
The magnitude of the input offset voltage can cause yield problems and can severely limit the applications in which the op amp can be used.
Input offset is not a serious problem in and of itself, since it can be cancelled by equal and opposite compensating signals. The problem with input offset is that it can change with temperature. This change in input offset voltage with temperature is called thermal drift. Thus, to maintain the performance of the op amp within specified criteria, the offset compensation mechanism must be correlated to thermal drift; that is, it must provide an offset cancellation signal that tracks the change in input offset voltage with variations in temperature.
The requirement for input offset compensation in bipolar op amp circuits is not so severe because it is possible to fabricate matching pairs of bipolar transistors with a relatively high level of precision. However, because of the well-known power consumption and speed benefits afforded by complementary metal-oxide semiconductor (CMOS) integrated circuits, this technology is becoming increasingly more popular for use in fabricating all types of circuits, including op amps. While CMOS op amps have relatively low input offset when operated in the sub-threshold region, input offset in these circuits when operated in or near the sub-threshold (weak inversion) or quasi sub-threshold (moderate inversion) region is dominated by the initial threshold voltage (V.sub.T).
The conventional technique for compensating for input offset and thermal drift in op amps is to provide a resistive nulling network that modifies the current relationships for the input differential pair.
FIG. 1 shows an op amp circuit in which the input stage is a differential PMOS pair that works into a bipolar load. Compensation for input offset voltage is provided by null resistors that provide an offset current to the differential pair for improved offset matching. That is, a correction voltage is reflected into the source leads of the input differential pair to balance the differential pair current densities.
The problem with this approach is that, as temperature changes, the trim input offset compensation current does not track the offset. That is, as temperature varies, the trim current and the offset error change at different rates.
When the op amp is operated in the sub-threshold region, the unbalanced currents magnify the offset error. That is, while the offset adjustment corrects for unbalanced differential pair currents, it doesn't correct for threshold voltage differences in the input FETs. Therefore, input offset voltage changes with changes in temperature.
Butler and Lane, "An Improved Performance MOS/Bipolar Op-Amp", 1974 IEEE International Solid State Circuits Conference, p. 138, describe an operational amplifier circuit, shown in FIG. 2, that also utilizes trimming current injection to compensate for input offset voltage. In this circuit, the correction voltage is derived from the IR drop in a series string of diffused resistors in which the temperature coefficient of the current is arranged to approximate the negative of the resistor temperature coefficient. Fine nulling is accomplished utilizing resistors R.sub.9 and R.sub.10, which are suitably tapped to minimize the effect of the diffused resister and trimpot temperature coefficients. However, this approach suffers from the same problems as the FIG. 1 offset compensation scheme in that the trim current does not track changes in the offset voltage with changes in temperature.
Thus, it would be highly desirable to have available a compensation circuit that can provide null input offset, but is not susceptible to thermal drift.
FIG. 3 shows a circuit described by Allen and Holberg, CMOS Analog Circuit Design, p. 248 for obtaining a negative temperature coefficient. The FIG. 3 threshold reference circuit balances threshold reference device Q1 against resistor R to provide a supply-independent reference current utilizable for setting up the bias of an op amp. However, this circuit has a strong negative temperature dependence.
Because the transconductance of MOSFETs decreases greatly at high temperature, a bias generator circuit that provides a positive temperature coefficient will offer more constant performance over a broad temperature range. The present invention optimizes op amp performance by using a positive temperature coefficient bias generator and a negative temperature coefficient offset trim current to maintain minimum offset drift with temperature.